The present invention relates to a flame retardant and smoke suppressed polymeric composition which is flame retardant and, even upon flaming, smoke suppressed and productive of no aggressive gases. The invention also relates to an electric wire having a sheath made from such a composition.
Recently, requirements for the flame retardancy of wirings for buildings and wirings in electrical instruments are getting more and more severe. Insulating materials for such writings are required to be not only flame retardant but also, even upon flaming, smoke suppressed and productive of no harmful gases that would exert adverse effects upon human body and the instruments. Resins containing halogen atoms in the molecular structure, such as polyvinyl chloride resins, cannot be used for this purpose since they evolve aggressive, harmful gases upon flaming. Halogen-free resins such as polyolefins which have generally incorporated therein a flame retarding amount of a halogen compound as a flame retardant additive are also productive of a halogen gases upon flaming. Thus, attempts to impart the flame retardancy to a halogen free resin by adding thereto a halogen free flame retardant have been proposed. Among others, hydroxides and carbonates of certain metals, such as aluminum hydroxide, magnesium hydroxide and magnesium carbonate, as well as zinc borate are well known in the art as a suitable halogen free flame retardant (see K. C. Hecker et al., Paper No. 17, ACS Division of Rubber Chem. April, 1972; I. Sobelev et al., SPE 31st. Ann. Tech. Conf. Montreal, 1973, Preprints, p. 709; and D. F. Lawson et al., Paper No. 13 ASC Division of Rubber Chem. October, 1974).
The flame retardancy of a polymeric material may be estimated by the oxygen index which may be determined, for example, by the procedure prescribed in JIS-K-7201. In order that a polymeric material can be rated as being flame retardant upon vertical burning tests it should have an oxygen index as high as 27 to 30 or higher. The amount of smoke upon flaming of a polymeric material may be estimated in terms of the maximum smoke density upon flaming of the material, which may be determined using suitable equipment known as an NBS smoke density chamber. It is generally desired to reduce the maximum smoke density of the material to a level of 100 or below.
It has been frequently experienced that when a halogen free flame retardant filler such as magnesium carbonate, magnesium hydroxide, aluminum hydroxide or zinc borate is added to a halogen free polymeric material such as a polyolefin to provide a halogen free polymeric composition which is satisfactorily flame retardant (e.g., having an oxygen index of 30 or higher) and simultaneously satisfactorily smoke suppressed (e.g., having a maximum smoke density, upon flaming, of 100 or below), the addition of an undesirably large amount of the filler, which adversely affects mechanical properties and processability of the resultant polymeric composition, is normally required.